This invention relates to preforms used in the production of metal matrix composites. In particular, this invention is concerned with low volume fraction preforms used in making metal matrix composites, especially light metal matrix composites of aluminum, magnesium, titanium and their alloys. The preforms of this invention are of particular interest in the pressureless infiltration and squeeze casting processes.
Many articles are formed from cast metal or metal alloys, and in particular from alloys of the so-called light metals, which includes aluminum, magnesium, and alloys thereof which often contain smaller proportions of several other elements. Typical alloying elements for magnesium include aluminum, beryllium, calcium, copper, lithium, manganese, metals from the rare earths group, silicon, silver, thorium, zinc, zirconium and yttrium; typical alloying elements for aluminium include silicon, iron, copper, manganese, magnesium, chromium, nickel, zinc, vanadium, titanium and gallium. Although these alloys are widely used, they do have certain disadvantages. Of particular importance are their inability to resist even moderately elevated temperatures, lack of inherent strength in comparison with metals such as ferrous alloys, and lack of adequate resistance to wear.
A need for lightweight, high strength parts by the aircraft and automotive industries, amongst others, has resulted in the development of reinforced metal matrix composites. In these metal matrix composites, a reinforcing phase is dispersed into the metal matrix, so that the composite more than compensates for the lack of mechanical, physical and other properties in the metal alone. The dispersed phase can be in the form of particles, whiskers, fibres, or, in the case of carbon or graphite fibre reinforcement, it can be tow. The reinforcement can be dispersed into the molten metal using a stir casting technique, or the reinforcement can be prepared as a preform, into which the molten metal is infiltrated, either by pressureless infiltration or by squeeze casting at a pressure of about 100 MPa. When a preform is used it must be capable of surviving the infiltration step more or less undamaged.
Typical procedures for making such composites are described by Lawrence, U.S. Pat. No. 3,529,655; Akiyama et al., U.S. Pat. No. 4,548,774; Tommis et al., U.S. Pat. No. 4,715,422; Corwin, U.S. Pat. No. 4,932,099; Jolly et al., U.S. Pat. No. 4,995,444; Wong, U.S. Pat. No. 5,360,662; Corbett et al., U.S. Pat. No. 5,458,181; Rohatgi, U.S. Pat. No. 5,711,362; Maier et al., CA 2,040,499; and Brown et al., CA 2,238,520. Typical materials used as the reinforcement in metal matrix Composites include the following:
in the from of fibers: silicon carbide, graphite, carbon, alumina, and mixtures of alumina and silica; PA1 in the form of whiskers: silicon carbide; PA1 in the from of tow: carbon and graphite; and PA1 in a particulate form: silicon carbide, carbon, alumina, titanium diboride, boron carbide and boron nitride. PA1 (a) mixing the reinforcement with a combination of sacrificial fillers, a sinterable binder, and sufficient liquid to provide a mouldable slurry; PA1 (b) placing the mouldable slurry into a preform mould; PA1 (c) curing the preform in the mould at a temperature and for a time sufficient to provide a dry green preform; PA1 (d) firing the green preform in a furnace controlled to provide a fired preform under the following combinations of time and temperature as a continuous sequence: PA1 cellulosic materials: corn starch; and PA1 inert plastics materials: powdered polyolefin, for example powdered polypropylene.
Although these metal matrix composites do overcome many of the problems associated with a comparable un-reinforced metal product, the metal matrix composites also suffer from several disadvantages which hinder their utilization. The reinforcement materials used, especially in the case of whiskers of material such as silicon carbide, are expensive. The metal matrix composites are often difficult to machine, and in many cases diamond cutting tools are required, thus again increasing manufacturing costs. The ductility properties utilized extensively in fabricating light metals, for example in making extrusions, are either impaired significantly, or effectively lost, if a relatively high volume fraction of reinforcement is used. In this context, the term "volume fraction" refers to the proportion of the volume of the metal matrix composite occupied by the reinforcement; for example, if the reinforcement volume fraction is 25%, one quarter of the volume of the composite comprises reinforcement, and three quarters comprises metal. A further difficulty is that there is a lack of reliable engineering data concerning metal matrix composites containing significant volume fractions of reinforcement, which is needed for structural design purposes.
These difficulties can be overcome to some degree by two expedients. In theory, it is possible to decrease the amount of reinforcement used by reinforcing only a selected part or parts of the metal matrix, for example an area exposed to significant stress, and it is also possible to decrease the amount of reinforcement by using a low volume fraction of reinforcement. By either of these techniques, or a combination of them, it is theoretically possible to fabricate a metal matrix composite product which consists primarily of metal, and in which either only selected regions include the reinforcement, or a relatively small amount of reinforcement sufficient is dispersed throughout the metal matrix, in both cases thereby to provide the required properties in the metal matrix composite. This should reduce the cost of the reinforcement used, and should result in a product that is machinable with conventional tooling, thus again lowering production costs. The structural properties of the metal matrix composite will then also approximate to those of the metal matrix, thus simplifying the design process, as the properties of the metals and alloys used in metal matrix composites are well understood.
There is still however a significant difficulty. In order to be able to use a low volume fraction of reinforcing material, either as a selective reinforcement at a particular locality, such as a point exposed to localised stress or wear, or distributed homogenously throughout a metal matrix, it is necessary to use the reinforcing material as a preform. In this context, the low volume fraction of reinforcing material is desirably typically below about 8% in the case of whisker reinforcement, and below about 15% in the case of particulate reinforcement. It also follows that in addition to corresponding to the required low volume fraction, the finished preform must have sufficient structural strength to be handled, and to survive the metal infiltration process, which often is a squeeze casting step carried out at a pressure of about 100 MPa.
Currently, the are no processes described whereby a reinforcement preform corresponding to this low volume fraction of the composite can be made in which the reinforcement is uniformly distributed. Uneven distribution of the reinforcement within a metal matrix composite, or even within a reinforced portion of a component, is not desirable for several reasons, not the least of which is that the properties of the metal matrix composite, or of the reinforced portion of a larger component, will vary in an unpredictable and largely random fashion.